The present invention relates to a control method for an induction motor, wherein a primary current thereof is resolved into a torque current component and an exciting current component which are separately controlled independent of each other for controlling relational speed of the induction motor.
In connection with the control of an induction motor, a vector control method has heretofore been known in which the primary current of the motor is resolved into a torque component and an excitation component which are separately controlled independent of each other. By adopting the vector control method, a speed control of the induction motor can be accomplished with a high speed response as in the case of a direct current motor.
The vector control method for the induction motor may be generally classified into a magnetic flux detection type vector control and a slip frequency control type vector control method. The former type control method is disclosed, for example, in U.S. Pat. No. 3,824,437 (corresponding to West-German Laid-Open (DOS) No. 1941312). A typical example of the latter type is disclosed in Japanese Patent Application Laid-Open No. 11125/1976. In the case of the magnetic flux detection type vector control, a magnetic flux detector has to be incorporated in the induction motor. Consequently, this type control method can not be employed in an induction motor designed for general purpose. Under the circumstances, the slip frequency control type vector control method has attracted attention in the recent years and is actually adopted in practical applications.
The slip frequency control type vector control method for the induction motor is based on the control of the output frequency of an inverter unit in dependence on the rotational speed of the motor. Consequently, signal conductors or cables are required for the output of a speed detector (or angular position detector) as well as for interconnection between the speed detector and the inverter unit, thus involving a complicated configuration of the system and troublesome procedure for application of this type of vector control method to the existing induction motor.
As an attempt to circumvent the disadvantages mentioned above, there has been proposed a so-called speed-sensorless vector control system for the induction motor in which no speed detector or sensor is employed and in which the motor current (primary current) and the frequency are controlled on the basis of the magnetic flux of the motor determined arithmetically from a terminal voltage of the motor, as is reported by Marian P. et al in their article titled "A Simple Control System for Current Source Inverter-Fed Induction Motor Drives" in IEEE (1983).
However, in the case of the speed-sensorless vector control system described in the literature cited above, there remain unsolved problems such as core saturation phenomenon in an insulating transformer employed for detecting the motor voltage, drift in an integrator for arithmetically determining or calculating the magentic flux and others, rendering it difficult or impossible to attain satisfactory accuracy in the calculation of the magnetic flux particularly in a low-frequency operation mode, thus stable operation cannot be assured.